The present invention relates to an image data storing device for an image forming apparatus and, more particularly, to an image data storing device to which image data are written by external equipment and from which the written data are read for forming an image.
A laser beam printer, or simply laser printer, is one of image forming apparatuses using the above-described type of image data storing device. Usually, in a laser printer, a laser beam issuing from from a semiconductor laser is collimated by a collimator lens to reach a scanning unit in the form of a rotary polygonal mirror. The polygonal mirror steers the incident laser beam toward an F-theta lens which in turn focuses it onto a charged surface of a photoconductive element. As the polygonal mirror is rotated, the beam spot formed on the photoconductive element is moved in the main scanning direction. At the same time, the photoconductive element is rotated to be thereby scanned in the subscanning direction by the beam spot. A photodector is located in a position outside of image data writing range of the photoconductive element. By detecting the laser beam deflected by the mirror, the photodector develops a synchronizing (sync) signal and delivers it to a signal processing circuit. In response to the sync signal, the signal processing circuit feeds incoming image data to a laser driver at predetermined timings. The laser driver drives the laser in response to the image data. As a result, a laser beam having been modulated by the image data scans the photoconductive element to electrostatically form a latent image thereon. The latent image is developed by a developing unit and then transferred to a paper sheet or similar recording medium. A horizontal sync signal derived from the sync signal outputted by the photodector is applied to external equipment.
The laser printer 1 constructed and operated as stated above does not supply the external equipment with information relating to the scanning speed thereof, i.e. the rotation speed and the number of surfaces of the polygonal mirror. Therefore, it is likely that the next one line of image data is sent from the external equipment to the printer before the latter fully writes the preceding line of image data, resulting in the conflict of image data. Should the write and read operations conflict with each other as mentioned, i.e., should one of them catch up with the other, signals would conflict on the I/O terminals of the line buffers to destroy a RAM included in the image data storing device and/or to cause the data to be lost.